1. Field of the Invention
The present invention relates to a lighting system for generating light by exciting a medium such as gas or vapor, and more particularly to a lighting system that can be used also as a lighting system in the ultraviolet, infrared, and far-infrared regions.
2. Description of the Related Art
There is generally known a gas laser as a lighting system for generating light by exciting a medium such as gas or vapor. In this specification, the “light” is not limited to visible light, but includes an electro-magnetic wave in a region other than the visible light (for example, the infrared region, far-infrared region, and ultraviolet region). More specifically, the “light” generated by the lighting system in this specification means an electro-magnetic wave having a wavelength that can be generated by medium excitation (the electro-magnetic wave depends upon the type of the medium).
Regarding techniques of exciting a medium in the gas laser or a laser medium, generally discharge excitation, electron beam excitation, and optical pumping techniques are already known as described, for example, in “Laser Handbook, 1st Edition, 2nd Printing (Chapter 15)/Issued by Ohmsha” (hereinafter, referred to as Nonpatent Document 1). As a technique using the discharge excitation among them, there is already known a gas laser disclosed, for example, in Japanese Patent Laid-Open No. Hei 1-200684 (hereinafter, referred to as Patent Document 1).
A gas laser using the discharge excitation generates a gaseous discharge from a discharge electrode into a laser tube to excite a laser medium by its discharge energy.
A gas laser using the electron beam excitation (for example, an excimer laser) applies a high voltage to a cathode (negative electrode) disposed in vacuum to release a beam of electrons accelerated at high speed (for example, electrons accelerated at 300 kV to 2 MV). It then causes the electron beam to impinge on a laser medium (KrF, XeF, or other gases) through a metallic thin film to excite the laser medium by the energy of the electron beam.
Moreover, a gas laser using the optical pumping uses a laser (CO2 laser and the like) as an excitation light source and excites a laser medium (CH3OH and the like) by means of the energy of the laser beam output from the excitation laser.
As additional information, there are already known various types of laser mediums for use in a gas laser (for example, refer to Appendix 4 of Nonpatent Document 1). The laser mediums include not only substances that are gaseous at room temperature (for example, He and Ne), but also substances that are solid or liquid at room temperature and are vaporized by heating at laser oscillation (for example, Cd and other metals, and H2O). The medium of the lighting system in this specification is a substance that is a gas or vapor at least when the light is generated (including mixed substances).
On the other hand, light in the far-infrared region (including a laser beam) is expected to be applied to imaging electro-magnetic waves that can be substituted for X rays and the demand for the lighting system in the far-infrared region are increasing. The far-infrared region generally ranges in wavelengths from 10 μm to 1000 μm.
In this instance, the gas laser can output a laser beam at various wavelengths including light in the far-infrared region according to the type of the medium (laser medium) and therefore in recent years it attracts attention as a lighting system in the THz-wave region (included in the far-infrared region).
Furthermore, as a simple and compact lighting system in the THz-wave region, a backward wave oscillator (BWO), namely a backward-wave tube is generally known as described on the Internet webpage “Tokyo Instruments, Inc., ‘Submillimeter Wave, Terahertz Products Lineup—Terahertz BWO Tube and Power Supply’” [online], [Searched for on Sep. 16, 2005] (URL: http://www.tokyoinst.co.jp/products/milli). This BWO releases an electron beam by applying a voltage to a heated cathode (negative electrode) and releases light in the THz-wave region by focusing the electron beam in the magnetic field and passing the focused electrons through a comb-like interdigital circuit.
Regarding the gas laser using the discharge excitation, particularly using metal vapor (for example, Cd, Zn, Se, or Cu vapor) as a laser medium (metal vapor laser), vaporized metal is deposited on the discharge electrode (negative or positive electrode) for causing a discharge, or sputtering of the discharge electrode occurs due to the metal vapor. They easily lead to deterioration or damage of the discharge electrode and a decrease in output power caused by contamination of the laser medium. Even in a gas laser not using metal vapor (for example, a He—Ne laser) among those using the discharge excitation, sputtering of the discharge electrode through an ionized gas occurs and it easily leads to deterioration or damage of the discharge electrode. Since the gas laser using the discharge excitation is susceptible to deterioration or damage of the discharge electrode as just described, it has required frequent replacement of a laser tube (gas tube) including the discharge electrode (for example, replacement after it has been used for 1000 hours). Moreover, the replacement has conventionally required a large cost in general. Furthermore, the metal vapor laser requires a heater for heating and vaporizing metal and therefore the gas laser tends to grow in size disadvantageously.
Regarding the gas laser using the electron beam excitation, there is a need to accelerate an electron beam at high speed to pass it through the metallic thin film and therefore it requires a high power supply, which is large in size and expensive. Furthermore, a gas pressure of the laser medium need be previously increased to some extent in order to facilitate the efficient energy reception from the electron beam to the laser medium. This causes a relatively large difference in pressure (for example, 1 atm pressure) between a vacuum chamber where the cathode is disposed and a gas chamber where the laser medium is enclosed. This difference in pressure easily deteriorates or damage the metallic thin film forming a partition wall between the vacuum chamber and the gas chamber. Consequently, it has required the frequent replacement of a gas tube including the metallic thin film.
Furthermore, regarding the gas laser using the optical pumping, a laser is used as an excitation light source and therefore the entire gas laser is large in size and expensive. In addition, the laser medium is excited by a laser beam, which leads to a low energy conversion efficiency disadvantageously. Moreover, the life of the laser as the excitation light source is as short as 1000 to 5000 hours or so Therefore, it has been required to frequently replace the excitation light source.
The conventional gas lasers have disadvantages that the life is relatively short, which easily leads to a high running cost. Furthermore, they also have a disadvantage such that the device configuration increases in size or cost according to the method of exciting the laser medium. This disadvantage similarly occurs in generating a laser beam in the THz-wave region by using a gas laser.
Moreover, although the BWO as a lighting system in the THz-wave region is simple and compact, it is necessary to apply a high voltage to the cathode to release electrons and to heat the cathode in order to release the electrons efficiently. For this reason, it is susceptible to deterioration or damage in the cathode. Therefore, the BWO has a disadvantage that the life is relatively short (for example, 500 to 1000 hours or so), which easily leads to high running cost disadvantageously, similarly to the gas lasers.
The present invention has been provided in view of the background described above. Therefore, it is an object of the present invention to provide a lighting system (including a gas laser) capable of emitting light efficiently with a compact and inexpensive device configuration and of achieving a long life. In addition, it is an object of the present invention to provide a lighting system also usable as a lighting system in the ultraviolet, infrared, and far-infrared regions.